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79

Information Notice No. 93­79: Core Shroud Cracking at Beltline Region

Welds in Boiling­Water Reactors

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555

September 30, 1993

NRC INFORMATION NOTICE 93‐79: CORE SHROUD CRACKING AT BELTLINE REGION WELDS

IN BOILING‐WATER REACTORS

Addressees

All holders of operating licenses or construction permits for boiling‐water

reactors (BWRs).

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information

notice to alert addressees that cracks have been observed in the weld regions

of the core support shroud in boiling water reactors. It is expected that

recipients will review the information for applicability to their facilities

and consider actions, as appropriate, to avoid similar problems. However,

suggestions contained in this information notice are not NRC requirements;

therefore, no specific action or written response is required.

Description of Circumstances

During the current refueling outage at Brunswick Unit 1 (a BWR‐4 reactor),

in‐vessel visual inspection revealed cracks at weld regions of the core

support shroud. The shroud is a stainless steel cylinder that serves to

direct the flow of water inside the reactor vessel. The shroud is completely

contained inside the 15.2 centimeter [6 inch] thick reactor vessel. The

structural integrity of the reactor vessel is not impacted by the cracks in

the shroud.

Carolina Power and Light Company (CP&L), the licensee for Brunswick, found

both circumferential and axial cracks in the shroud. The circumferential

cracks were located in the inside shroud surface in the heat‐affected zone

(HAZ) of weld H‐3 and extended 360 degrees around the circumference of the

shroud (see Figures 1 and 2). Weld H‐3 is a horizontal weld which fuses the

top guide support ring to the lower shroud. The first axial crack discovered

was located on the outside shroud surface of weld H‐4 in the lower shroud.

CP&L performed additional visual testing (VT) and ultrasonic testing (UT) of

the shroud and removed boat samples to evaluate the length and size of the

cracks.

Discussion
In 1990, crack indications were reported at core shroud welds located in the

beltline region of an overseas reactor (BWR4). This reactor had completed

approximately 190 months of power operation before the cracks were discovered.

As a result of this discovery, General Electric (GE) issued Rapid Information

Communication Services Information Letter (RICSIL) 054, "Core Support Shroud

9309290050.

IN 93‐79

September 30, 1993

Page 2 of 3

Crack Indications," on October 3, 1990, to all owners of GE BWRs. The RICSIL

summarized the cracking found in the overseas reactor and recommended that at

the next refueling outage plants with high‐carbon‐type 304 stainless steel

shrouds perform a visual examination of the accessible areas of the seam welds

and associated HAZ on the inside and outside surfaces of the shroud.

Since early July, CP&L has performed VT inspections of the Unit 1 inside and

outside shroud surface in the vicinity of welds. These inspections were

performed in accordance with GE RICSIL 054 and discovered cracks in the weld

regions. CP&L determined that in order to perform an adequate VT it was

necessary to remove the outer blade guides, pre‐clean inspection areas, and

obtain an improved resolution of "1‐millimeter wire" (in lieu of the

Code‐prescribed resolution). Camera and lighting positions were also found to

be crucial in performing adequate VTs. Also, CP&L has worked with GE to

develop more sophisticated UT equipment to identify how deeply into the shroud

metal the crack extends.

Additional VT inspections revealed more axial cracks at the inside surface of

weld H‐4 as well as cracks at welds H‐1, H‐2, and H‐5 of the shroud. One of

the additional cracks, a circumferential crack at weld H‐5, appears to be

approximately 76.2 centimeters [30 inches] in length. The majority of the

cracks are located in the HAZ of the welds, although one crack was discovered

in the central region of shroud plate P‐6. The crack in P‐6, however, may be

associated with a possible weld repair of a surface defect in the plate after

its fabrication at the mill.

The results from the boat samples indicated intergranular stress‐corrosion

cracking (IGSCC) as the mechanism. Preliminary results suggest that the crack

in the HAZ of weld H‐3 may be 3.8 centimeters [1.5 inches] or more in depth.

The location of this crack is shown in Figure 2.

As a result of the shroud cracks being discovered on Unit 1, CP&L re‐examined

the results of the inspection performed during the 1991 refueling outage of

Unit 2. The re‐examination revealed three minor crack indications in the HAZ

of weld H‐2. The licensee concluded that the cracks do not pose a concern to

normal operation of the reactor.

CP&L plans to repair the Brunswick Unit 1 core shroud before the plant is

brought back into service. CP&L intends to restore the strength of the shroud

by adding stiffening braces around the top portion of the shroud. However,

the licensee will continue to examine and evaluate the cracks in the core

shroud.

General Electric issued Revision 1 to RICSIL 054 on July 21, 1993, to update

the information on the core support shroud cracks and to provide revised

interim recommendations to perform visual examination of accessible areas of

the shroud at all GE BWRs during the next scheduled outage. The NRC has been

informed by GE that they are evaluating the Brunswick results and will provide

.

IN 93‐79

September 30, 1993
Page 3 of 3

updated information to owners of GE BWRs. The NRC staff is evaluating the

implications of the shroud cracks for reactor core configuration and emergency

core cooling system performance under accident conditions at operating plants

and will consider the need for additional generic communications.

This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

one of the technical contacts listed below or the appropriate Office of

Nuclear Reactor Regulation (NRR) project manager.

/s/'d by BKGrimes

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contacts: R. A. Hermann, NRR J. Medoff, NRR

(301) 504‐2768 (301) 504‐2715

P. Byron, Region II T. Greene, NRR

(919) 457‐9531 (301) 504‐1175

Attachments:

1. Figure 1: Weld and Plate Locations in the Beltline

Region of the Brunswick Unit 1 Core Shroud

2. Figure 2: Details of Weld Locations H‐2 and H‐3 in the

Brunswick Unit 1 Core Shroud

3. List of Recently Issued NRC Information Notices

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